1. Field of the Invention
The present invention relates to a photosensitive member for electrophotography of the type which comprises a photoconductive layer containing photoconductive cadmium sulfide particles dispersed in a binder. More particularly, the present invention is directed to improvements in the properties of such photosensitive member by adding a particular additive to the photoconductive layer. According to the invention, the additive is selected from the group consisting of iron, nickel, cobalt and compounds of said metals. The photosensitive member containing such additive is less affected by the conditions under which it was kept before use and therefore unfavorable variation in contrast of electrostatic images produced through the photosensitive member is substantially reduced.
2. Description of the Prior Art
In the art, various types of photosensitive members have been known and used. The construction which a particular photosensitive member has to have is determined depending upon the desired properties for the photosensitive member as well as the type of electrophotographic process in which the photosensitive member is applied.
One typical photosensitive member for electrophotography hitherto widely used has a structure comprising a substrate and a photoconductive layer formed on the substrate. Another typical example is of the type which comprises a top layer of an electrically insulating layer in addition to the substrate and photoconductive layer. The photosensitive member comprising a substrate and a photoconductive layer is used for forming images in accordance with the commonest electrophotographic process comprising the steps of charging, imagewise exposure, developing and, if necessary, transferring. The photosensitive member having an insulating top layer mentioned above is used also in the same electrophotographic process and is useful for other particular electrophotographic process. The function of the insulating layer is to protect the photoconductive layer, to improve the mechanical strength and dark decay of the photosensitive member and also to prevent environmental pollution.
Examples of such photosensitive member having an insulating layer in addition to substrate and a photoconductive layer and examples of electrophotographic processes employing such photosensitive member are disclosed, for example, in U.S. Pat. Nos. 2,860,048; 3,146,145; 3,607,258; 3,666,363; 3,734,609; 3,457,070; 3,124,456 and Japanese Patent Publication No. 16,429/1966.
As a matter of course, a photoconductive material used for electrophotography is required to have a predetermined sensitivity, electrical property and also optical property appropriate for the electrophotographic process in which the material is intended for use.
One of the most important factors by which the properties of a photosensitive member are determined is photoconductive material. The material most frequently used as a photoconductive material for electrophotography is pulverized cadmium sulfide. For cadmium sulfide particles commonly used for electrophotography it is essential to adequately control the amount of doping impurity, condition for precipitation, condition for calcination which is carried out to disperse the doped impurity and also condition for after-treatment, since the surface characteristics of formed cadmium sulfide particles are variable depending upon these manufacturing conditions. However, it is not easy to adequately control these conditions and to obtain cadmium sulfide particles having desired surface characteristics. The property of cadmium sulfide for electrophotography is much more affected by surface characteristics of particles rather than by bulk characteristics of particles. Under above-mentioned conditions there is sometimes produced such cadmium sulfide having many traps formed on the particle surface. When such cadmium sulfide is used to form a photosensitive member, the photosensitive medium will show the following drawbacks. One of the drawbacks is found in that the photoconductive layer, when actually used for image formation, exhibits variation in resistance according to the condition under which the photosensitive member was kept. Namely, the photoconductive layer has different resistances according to whether is has been exposed to light or not during storage. Another drawback is that the light photodecay thereof is also variable according to the storage condition.
Hitherto the above mentioned drawbacks have been considered not to be of critical importance. However, with the great advancement of image forming technique in these years the drawbacks appears as an important problem against further improvement of image quality of copies. To satisfy the desire for higher image quality, the difference in resistance and photodecay mentioned above must be minimized. In other words, it is essential that the cadmium sulfide itself has a high resistance and the photosensitive member containing the cadmium sulfide exhibits always stable and rapid photodecay irrespective of storage conditions.
Conventionally it has been tried to obtain the finally desired properties of the photosensitive member by
1. changing the amount of impurity for doping and
2. changing the conditions for sintering.
However, these measures can not always give the desired properties.